The present invention is directed generally to gain control techniques, and is directed more particularly to a switched gain control network which substantially reduces noise due to the switching signals used to control the network.
In RF applications, such as radar receivers, gain control elements such as gain control blocks or transfer switches are utilized to control the level of the input signal to an analog-to-digital (A/D) converter. Generally, a gain control block is a circuit with two RF paths between its input and its output, wherein one path provides no attenuation or amplification while the other path provides attenuation or amplification. The path that is enabled is selected by a switching command signal to the gain control block.
Gain control blocks are utilized in radar systems for the following reasons.
In a radar system, a pulse, modulated with a carrier, is transmitted, and by observing the characteristics of the return pulse, information about the target, free space, or ground objects can be deciphered. The amplitude of the return pulse varies in proportion with the distance between the radar antenna and the object. In order to maintain a constant amplitude at the input of the an A/D (analog-to-digital) converter, and to prevent saturation of associated circuitry, gain control circuitry is used to automatically adjust the output to a constant level. In a modern radar system, like the ones used in fighter aircrafts, the gain controlling function is achieved by a digital command received from an on-board computer which monitors the amplitude of the pulse near the input of the A/D converter. Operating in closed loop environment, this digital command is automatically adjusted such that the amplitude of the pulse is maintained at a constant level.
Serially connected gain control blocks are commonly utilized to provide the desired level control pursuant to respective control signals that are selected to provide different amounts of attenuation or amplification. For example, such gain control blocks can be binarily weighted in dBs so that integral multiples of the least attenuation or amplification can be selected.
A consideration with the use of gain control blocks as well as other gain controlling switching circuitry are the transient "glitches" or spikes that appear on the input signal being controlled due to the switching of the gain control blocks. The glitches propogate through the gain control blocks and are ultimately amplified, appearing at the amplifier output as unwanted distortion. Efforts to control the transient glitches due to switching have included filtering of the command pulses. However, since filtering reduces bandwidth and increases switching time, filtering results in slower switching speeds.